Shihua Fu

Manipulation of aerosols revolving in taper-ring optical traps.

We designed taper-ring optical traps by a weakly focused laser beam through a circular aperture. By railing-like potential barriers, these optical traps are partitioned into enclosed rings, in which irregular light-absorbing microparticles can be driven by photophoretic force to revolve around optical axis in air. The diameter of revolution can reach about 700 μm, which is much larger than that in traditional optical traps based on radiation pressure and gradient force. More importantly, multiple particles were driven to revolve simultaneously in different planes in air for the first reported time to the best of our knowledge.

3D digital image correlation investigation of PLC effect in a new Ni–Co base superalloy

Repeated plastic instability accompanying serrated yielding in stress–strain curves and localization of deformation is observed during plastic deformation of many metallic alloys when tensile specimens are deformed under certain experimental conditions of temperature, strain rate, and pre-deformation. This phenomenon is referred to as the Portevin- Le Chatelier (PLC) effect. TMW alloy, a newly developed Ni–Co base superalloy for aircraft engine application, also exhibit PLC effect during tensile test at temperatures ranging from 300 ℃ to 600 ℃, which are also the temperature range for engine working. In this paper, a 3D digital image correlation (3D DIC) measurement system was established to observe the localization of deformation (PLC band) in a tensile test performed on TMW alloy specimen at temperature of 400 ℃. The 3D DIC system, with displacement measurement accuracy up to 0.01 pixels and strain measurement accuracy up to 100 με, has a high performance in displacement field calculation with more than 10000 points every second on a 3.1G Hz CPU computer. The test result shows that, the PLC bands are inclined at an angle of about 60° to the tensile axis. Unlike tensile test performed on aluminums alloy, the widths of PLC bands of TMW alloy specimen, ranging from 4 mm to 4.5 mm, are much greater than the specimen thickness (0.25 mm).

The Mechanism of Critical Strain of Serrated Yielding in Strain Rate Domain

The mechanism of the critical strain of serrated yielding is studied via tension tests at various strain rates. Before the critical strain, it is deduced that dislocations are not pinned at high strain rates, and dislocations at low strain rates are pinned but cannot escape. The critical strain depends on the first pinning process at high strain rates and on the first unpinning process at low strain rates. The calculated results based on the two criteria are in good consistency with the experiment.

Influence of precipitation on the Portevin-Le Chatelier effect in Al-Mg alloys

Abstract In the alloy with solute content higher than the limiting solubility, the solute atoms that have failed to dissolve will precipitate from the solid solution and form precipitations. In this study, the Portevin-Le Chatelier (PLC) effects in annealed 5456 and 5052 aluminum alloys with different precipitation contents have been investigated under different applied strain rates. The results suggest that precipitations have significant effect on the PLC effect and the more the precipitations are, the greater the influence is. Furthermore, the solute diffusion is pipe diffusion in 5052 alloy with lower precipitation content. However, for 5456 alloy with higher precipitation content, the diffusion is no longer the case but more complex.

SIMULATION OF DYNAMIC STRAIN AGING PROCESS AT THE MICROSCOPIC SCALE BY MONTE CARLO DYNAMIC MODEL

A two-dimensional Monte Carlo dynamic model has been developed to simulate the interaction between mobile dislocations and solute atoms. When there was a single dislocation with a constant stress rate, the dislocation was pinned under low stress rate conditions, moved continuously under high stress rate conditions, and moved intermittently at an intermediate stress rate, where the step-shaped curve of dislocation displacement showed dynamic strain aging. Under multi-dislocation unstressed conditions, the solute atoms gathered below positive dislocations and above negative dislocations. Under the multi-dislocation conditions with constant stress, the dislocation displacement became longer with increasing stress. Under multi-dislocation conditions with constant stress rate, the collective pinning and unpinning results showed a step-shaped curve of total dislocation displacement. The simulation results present a microscopic view of the pinning and unpinning in dynamic strain aging.